chore: migrate package management to npm

[Che-Zhu]

Summary

• replace pnpm/Corepack with npm across local development, CI, and Docker
• migrate pnpm-lock.yaml to npm lockfile v3 without changing direct dependency versions
• move pnpm overrides to npm overrides
• document npm ci for reproducible clean installs
• keep Node.js 22.12 and npm 10 as the supported baseline

Why

Fulling is currently a single-package application and does not rely on pnpm-specific workspace features. Using the package manager bundled with Node.js removes the Corepack and pnpm bootstrap from contributor machines, CI, and container builds while retaining deterministic installs through npm ci.

Runtime and dependency upgrades are intentionally excluded and tracked separately in #183.

Validation

• npm ci --ignore-scripts on Node.js 22.12 / npm 10
• npm run lint
• npm test — 35 tests passed
• npm run test:e2e — 18 tests passed
• npm run build
• Docker image build, startup, and HTTP 200 smoke test
• direct dependency versions compared against the previous pnpm lockfile

Closes #180